1. Field
The present disclosure relates to the manufacture of light emitting devices, and more particularly, to the manufacture of broad-spectrum light emitting diodes having a phosphor layer.
2. Background
Solid state devices, such as light emitting diodes (LED)s, are attractive candidates for replacing conventional light sources such as incandescent and fluorescent lamps. LEDs have substantially higher light conversion efficiencies than incandescent lamps and longer lifetimes than both types of conventional light sources. In addition, some types of LEDs now have higher conversion efficiencies than fluorescent light sources and still higher conversion efficiencies have been demonstrated in the laboratory. Finally, LEDs require lower voltages than fluorescent lamps, and therefore, provide various power saving benefits.
Unfortunately, LEDs produce light in a relatively narrow spectrum. To replace conventional lighting systems, LED-based sources that produce white light are desired. One way to produce white light is to deposit a phosphor material on the LEDs, such that monochromatic light emitted from blue or UV LEDs is converted to broad-spectrum white light. The phosphor material may be formed by mixing a phosphor powder into a polymer such as silicone at a pre-defined concentration, or with a pre-defined recipe, resulting in a suspension of phosphor particles in the silicone. This mixture is then deposited onto the LED at a pre-defined volume and/or weight, and subsequently subjected to a curing procedure. The resulting phosphor-coated LEDs are then tested and put into different color bins according to the actual tested color. Various processes for suspending phosphor particles in silicone carriers are known in the art.
Using these processes, it is difficult to achieve consistent optical properties, such as color consistency. Often, due to the process of suspending the phosphor particles in the carrier, the uniformity of light across large numbers of LEDs is difficult to maintain. Operator error may result in the wrong mixture, leading to an off-color failure of the whole lot. Further, the viscosity of the phosphor mixture may change during deposition, or the phosphor suspension may settle at pot life, causing a wide range of color bins. Moreover, other factors such as the chip wavelength, the phosphor profile, the substrate reflectivity, etc., can also cause variation even when the dispensing volume and weight are consistent. The above issues generally can not be caught in real-time, and thus, when they are discovered during testing it is too late to recover the parts and they must be scrapped. The manufacturing process itself is often time consuming and costly, requiring multiple fabrication steps to complete. All of these issues lead to increased cost in manufacturing of broad-spectrum LEDs.
Accordingly, there is a need in the art for simplified and improved processes for applying a phosphor material to LEDs and other solid state lighting devices.